The present invention relates to a method and a device for adjusting a battery module including connecting cells, which are nickel-metal hydride batteries, to reduce variations, resulting from usage, in the state of charge at the positive electrodes of the cells.
A nickel-metal hydride battery is a rechargeable battery that uses nickel hydroxide for the positive electrode, a hydrogen adsorption alloy for the negative electrode, and an alkali solution such as a sodium hydroxide solution for the electrolytic solution. Generally, in a nickel-metal hydride battery, the negative electrode has a greater capacity than the positive electrode so that the capacity of the positive electrode restricts the discharge capacity of the entire battery. This is referred to as positive electrode restriction. Positive electrode restriction suppresses internal pressure increases when the battery is over-discharged or over-charged. When the state of charge (SOC) at the positive electrode is 100%, the charge capacitance remaining at the negative electrode is referred to as a charge reserve. When the state of charge (SOC) at the positive electrode is 0%, the discharge capacitance remaining at the negative electrode is referred to as a discharge reserve. The state of charge of the positive electrode refers to the ratio of a charged active material amount relative to the entire active material amount contained in the positive electrode. Thus, when a positive electrode SOC is 0%, the ratio of the charged active material is 0%.
In a battery module that connects a plurality of cells, in cell temperature differences may result in the cells having different charging efficiencies. Continuous use of the battery module with each cell having a different charging efficiency would gradually result in the cells having different states of charge. When the battery module is discharged under such a condition, the cell with the positive electrode of which SOC becomes 0% most quickly restricts discharging in the other cells. This virtually decreases the entire discharge capacity. Accordingly, it is desirable that variations in the SOC at the positive electrodes of the cells be adjusted and reduced so that the capacity of each nickel-metal hydride battery approaches the initial capacity.
Japanese Laid-Open Patent Publication No. 2004-319366 describes an example of a method for restoring the capacity of a nickel-metal hydride battery by drawing hydrogen into a battery case or drawing an alkali metal or the like that reacts with the electrolytic solution and generates hydrogen into the battery case.